73 disc brake install on 68 Dart

Just about finished mounted calipers in rear and had some brake lines made up.

Now I need a 73 disc/drum master cylinder and no one has one in stock. Thinking about one off a 1983 Diplomat everything looks the same bore and port size. Just need an adapter to mount on firewall. My concern is the original on the dart has that retaining clip that bolts to the master keeping the rod in. Not sure if that is needed or required for the diplomat master cylinder. See pictures on original and Diplomat style master. Will the rod be long enough after the adapter is installed?